Note added 30 April 2007: Until now on the Forum this species has been considered a form of C. sinuata [see message #19854]. It is now recognised as C. flavicostata.

Thanks Sabine,
You certainly saw it in a hurry - definitely moving more like a flatworm than a slug. Concerning how many species in the Ceratosoma sinuata / flavicostata group, I'm afraid I have nothing to report.
Best wishes,
Bill Rudman